(1) Field of the Invention
The present invention relates to an optical switching system for routing optical signals that contain address information indicating destinations of the optical signals in an optical communication network.
(2) Description of the Related Art
In optical communication networks, switching apparatuses are placed at switching points where they receive optical signals from optical transmission apparatuses and switch the destinations of the received signals. At the switching points, the switching apparatuses route the optical signals according to the address information contained in packets that constitute the optical signals, where each signal, either electrical or optical, is a sequence of packets. The electric signals are converted into the optical signals through a modulation and then an electrical-to-optical conversion. Each of the packets constituting the signals is composed of transmission data and a header that contains address information indicating the destination of the signal. The packets constituting the optical signals are hereinafter referred to as optical packets.
In earlier days, when conventional switching apparatuses receive an optical signal composed of optical packets, they convert each optical packet into an electric-signal packet by performing an optical-to-electrical conversion and then a demodulation so as to extract the address information from the header of each electric-signal packet.
The conventional switching apparatuses then update the address information if necessary, re-convert the electric signal into the optical signal, and transmit the re-converted optical signal to the destination (another switching apparatus or a destination apparatus) in accordance with the extracted address information (see Patent Document 1: Optical Label Multiplexing, FIG. 4, listed below).
As understood from the above description, each switching apparatus performs the two-way conversion, namely optical-to-electrical and electrical-to-optical, to extract the address information from each packet. As a result, the conventional system has a problem that the transmission speed in the optical communication network is limited by the two-way conversion performed by the switching apparatuses.
For the purpose of solving the above-mentioned problem, various methods based on the optical label switching have been proposed so far (see, for example, the Patent Document 1 (especially FIG. 6) and Non-Patent Documents 1 and 2, listed below).
In the optical label switching, an optical transmitting unit transmits an optical signal that contains the address information (hereinafter referred to as a label) in each header of each packet, and an optical switching unit that receives the optical signal causes the received optical signal to branch into two optical signals that have the same information, extracts and extracts only the label of each packet contained in one branched optical signal, and outputs the other branched optical signal after switching its route based on each extracted label.
As described above, in the known optical label switching, the optical switching unit need not convert optical signals of the entire packet into electrical, but reads only a part (the label) of each packet. Further, the optical switching unit need not re-convert the electric signal into an optical signal after it extracts the label. Such an arrangement in this technology is considered to have increased the speed in switching the destinations of the optical signals.
However, in this known optical label switching method, the optical transmitting unit needs to perform a modulation and a frequency conversion on a label signal that is generated based on the label, so that the label signal has a higher frequency than the other portions of the electric signal. The optical switching unit performs a frequency conversion and a demodulation on the received label signal to extract the label.
Accordingly, the optical signal switching speed at the optical switching unit is limited by the modulation, demodulation, and frequency conversion of the label signal.
The above-mentioned problem in the known optical label switching will be described in detail with reference to FIGS. 9-11.
Known Optical Label Switching Method
FIG. 9 is a functional block diagram showing the construction of an optical transmitting unit included in an optical label switching system.
FIG. 10 is a functional block diagram showing the construction of an optical switching unit included in the optical label switching system.
An optical transmitting unit 60 converts data into an optical signal and transmits the optical signal into an optical communication network as a sequence of optical packets.
An optical switching unit 61 assigns each optical packet it receives from the optical transmitting unit 60 to an appropriate port in accordance with the destination of the optical packet, and outputs the optical packet to the assigned port.
First, the operation of the optical transmitting unit 60 will be explained.
A data signal generating unit 604 generates a data signal representing data to be transmitted, and outputs the generated data signal.
A label generating unit 601 generates a label signal based on the label contained in each packet, where the label indicates the address information of the packet constituting the data signal.
A label modulation unit 602 modulates the label signal and outputs a modulated label signal. The label modulation unit 602 is achieved by, for example, a BPSK (Binary Phase Shift Keying) modulator.
FIG. 11 shows the frequency allocation of the data signal and the label signal. Since the label signal is a modulated signal, it has a predetermined band width, with its center at f0.
A frequency conversion unit 603 converts the frequency of the modulated label signal into a desired frequency, and outputs the modulated label signal having the desired frequency.
An electrical-to-optical conversion unit 605 outputs a continuous optical signal.
An external modulation unit 606 performs the intensity modulation on the continuous optical signal received from the electrical-to-optical conversion unit 605 in accordance with (i) the modulated label signal received from the frequency conversion unit 603 and (ii) the data signal received from the data signal generating unit 604, and outputs an optical signal composed of packets.
An optical switching unit 61 extracts a label contained in an optical signal that has been received via an optical line 11, selects one among n output ports based on the extracted label, and outputs the optical signal to the selected port, where n is an integer no smaller than xe2x80x9c2xe2x80x9d.
As shown in FIG. 10, an optical branching unit 62 of the optical switching unit 61 receives the optical signal via the optical line 11, and causes the received optical signal to branch to a label identifying unit 63 and a port switching unit 64.
An optical-to-electrical conversion unit 631 of the label identifying unit 63 receives one of the outputs from the optical branching unit 62, and converts the branched optical signal into an electric signal.
A filter 632 extracts a modulated label signal from the electric signal output from the optical-to-electrical conversion unit 631.
A frequency conversion unit 633 converts the frequency of the modulated label signal.
A label demodulation unit 634 demodulates the electric signal output from the frequency conversion unit 633, and outputs a demodulated label signal.
A label detecting unit 635 detects the address information, namely the label, from the label signal output from the label demodulation unit 634.
A port control unit 636 determines a port among the n output ports based on the address information detected by the label detecting unit 635, and controls the port switching unit 64 to output the other branched optical signal to the determined port.
As described above, in the known optical label switching method, the optical transmitting unit performs a modulation and a frequency conversion on a label signal, and the optical switching unit performs a frequency conversion and a demodulation on the received label signal.
Accordingly, the optical signal switching speed at the optical switching unit is limited by the modulation, demodulation, and frequency conversion of the label signal.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-36477.
Non-Patent Document 1: S. J. Ben Yoo, xe2x80x9cAll-optical label switching: Architecture, Protocol, and Systems Demonstrationxe2x80x9d, Technical Digest of OptoElectronics and Communications Conference (OECC), paper no. 9A1-2, pp. 8-9, Yokohama, July, 2002.
Non-Patent Document 2: A. B. Sahin et al., xe2x80x9cDispersion Division Multiplexing for In-Band Subcarrier-Header-Based All-Optical Packet Switchingxe2x80x9d, OSA Technical Digest of OFC (Optical Fiber Communication conference), no. WO1, pp. 279-280, March, 2002.
Non-Patent Document 3: Iga (writer and editor), xe2x80x9cSemiconductor Laserxe2x80x9d, Chapter 7: Direct Modulation and Pulse Generation, Ohmsha (book publishing company), 1994.
Non-Patent Document 4: Ikushima et al., xe2x80x9cStudy on Analog Performance of optical Devices and System Design for 10,000 ch WDM/SCM Optical Transmission System using Direct Modulationxe2x80x9d, ITE (The Institute of Image Information and Television Engineers) Journal, vol.55, No.10, 2001, pp.1315-1321.
The object of the present invention is therefore to provide an optical switching system that has a simple circuit construction without circuits for modulation/demodulation and frequency conversions that are required in the known optical label switching, and switches the route of the optical signal at a high speed.
The above object is fulfilled by an optical switching system, comprising: a signal input unit operable to input an optical packet signal that includes an optical label signal converted by a direct modulation method from an electric label signal that has a frequency that corresponds to a destination of the optical packet signal; an optical-to-electrical conversion unit operable to convert the optical packet signal, which has passed through an optical filter that allows only signals with wavelengths within a predetermined range to pass through itself, into an electric signal; an extracting unit operable to extract the electric label signal from the electric signal; an electric power level detecting unit operable to detect an electric power level of the extracted electric label signal; a port determining unit operable to determine, based on the detected electric power level, a port to which the optical packet signal is to be output; and a signal output unit operable to output the optical packet signal to the determined port.
With the above-stated construction, the optical switching unit can identify the address information of an optical packet signal containing a label signal by detecting the electric power level of the label signal having been caused to pass through an optical filter and then been subjected to an optical-to-electrical conversion. Such a circuit requires only a simple construction and switches the route of the optical packet signal at a high speed without being limited by the processing speeds of circuits for modulation/demodulation and frequency conversions.
The optical packet signal dealt with in the optical switching system (i) includes an optical label signal converted by a direct modulation method from an electric label signal having one of frequencies that are assigned to a plurality of routes in advance, or (ii) has been converted by a direct modulation method from a multiplex signal in which the data signal is multiplexed with the label signal having one of frequencies that are assigned to a plurality of routes in advance.
Conventional label signals are modulated signals. Such conventional label signals have a problem that the changes in amplitude of the modulated signals affect the data signals. In contrast, the optical switching system of the present invention has an advantageous effect that the label signals used therein do not affect the data signals.
The above object is also fulfilled by an optical switching system, comprising: a signal input unit operable to input an optical packet signal that includes an optical label signal generated by phase-modulating an optical signal based on an electric label signal that has a frequency that corresponds to a destination of the optical packet signal; an optical-to-electrical conversion unit operable to convert the optical packet signal, which has passed through an optical filter that allows only signals with wavelengths within a predetermined range to pass through itself, into an electric signal; an extracting unit operable to extract the electric label signal from the electric signal; an electric power level detecting unit operable to detect an electric power level of the extracted electric label signal; a port determining unit operable to determine, based on the detected electric power level, a port to which the optical packet signal is to be output; and a signal output unit operable to output the optical packet signal to the determined port.
With the above-stated construction, the optical switching unit can identify the route of an optical packet signal containing an optical label signal generated by phase-modulating an optical signal based on an electric label signal that has a frequency that corresponds to a route of the optical packet signal. Such a circuit requires only a simple construction and switches the route of the optical packet signal at a high speed.
In the above-stated optical switching system, the electric label signal may have a sinusoidal waveform.